Projection texture painting, is it possible to swap images after applying brush?
I would suggest using 3 uv maps, on for front projection, one for side projection, and one for regular unwrap with seams to avoid stretch. Here each texture is set to a different slot, and a black image is set as stencil and intensity between them, and that one is painted with white to reveal the image below. In this case, you can then replace the texture use for the front view or the side view and the stencil will affect it the same way.